Physiology of Finfish and Shellfish (2+1)

3.1.34. Embryonic Development in Fish

Embryonic development starts with eggs

The initial stages of embryonic development are almost identical for both lower and higher level invertebrates. Fish embryonic development consists of seven stages leading to hatching. These stages are the zygote period, cleavage period, blastula period, gastrula period, segmentation period, pharyngula period, and finally hatching.

Inception

Fish embryonic development begins after inception with the zygote period. During this time the embryo develops one cell that looks like a half bubble. This lasts on average for 45 minutes with a cell .7 mm in diameter. The chorion, also known as the egg shell, will swell and lift away from the fertilized egg.

Cleavage Period

The next stage of embryonic development allows the fish to rapidly accumulate new cells. As cleavage occurs, the cells will divide at 15-minute intervals. Depending on the complexity of the fish, this cycle will last 45 minutes to 2 1/2 hours.

Gastrula and Pharyngula Periods

Gastrula period will last anywhere from 5 1/4 to 10 hours. During this stage organs will begin to become visible, and the developing tail becomes more prominent. The embryonic structure also elongates. Next the pharyngula period occurs, where the fish will begin to exhibit fully formed organs. This can last anywhere from 24 to 48 hours. It is during this period that the distinction between upper and lower vertebrate embryonic development can be made.

Hatching

During the next 48 to 72 hours the external indicators of the fish develop. Gills, jaw and pectoral fins grow at an accelerated rate. Once development is complete the tiny fish is ready to hatch. Over the next several days the fish larvae will emerge from their eggs at various rates, ready to hide and grow in their new environment.